A value of type 'a Lazy.t is a deferred computation, called
a suspension, that has a result of type 'a. The special
expression syntax lazy (expr) makes a suspension of the
computation of expr, without computing expr itself yet.
"Forcing" the suspension will then compute expr and return its
result.

Note: lazy_t is the built-in type constructor used by the compiler
for the lazy keyword. You should not use it directly. Always use
Lazy.t instead.

Note: if the program is compiled with the -rectypes option,
ill-founded recursive definitions of the form letrec x = lazy x
or letrec x = lazy(lazy(...(lazy x))) are accepted by the type-checker
and lead, when forced, to ill-formed values that trigger infinite
loops in the garbage collector and other parts of the run-time system.
Without the -rectypes option, such ill-founded recursive definitions
are rejected by the type-checker.

force x forces the suspension x and returns its result.
If x has already been forced, Lazy.force x returns the
same value again without recomputing it. If it raised an exception,
the same exception is raised again.
Raise Undefined if the forcing of x tries to force x itself
recursively.

force_val x forces the suspension x and returns its
result. If x has already been forced, force_val x
returns the same value again without recomputing it.
Raise Undefined if the forcing of x tries to force x itself
recursively.
If the computation of x raises an exception, it is unspecified
whether force_val x raises the same exception or Undefined.